Switching power converters offer both compactness and efficiency in a number of different topologies that can be placed in two main categories: isolated (or transformer-coupled) and non-isolated (or direct-coupled). In non-isolated switching power converters, such as a buck (reducing voltage) or boost (increasing voltage) converter, the power output is directly coupled to the power input through the power switch element. In contrast, in isolated power converters, such as flyback or forward converters, the power output is isolated from the power input through a transformer, with the power switch element located on the primary (input) side of the transformer.
In either type of converter, typical analog control systems use pulse width modulation (PWM), pulse frequency modulation (PFM), or a combination thereof to control the duty cycle of the power switch within the converter.
Consider, for example, the flyback converter 10 of FIG. 1. The converter 10 includes a power switch Q1 (typically a field effect transistor (FET)) coupled to an input voltage, Vin, via a primary winding 20 of a power transformer T1. A rectifying diode D1 and filter capacitor C1 are coupled to a secondary winding 22 of the transformer T1. The converter 10 includes a pulse modulating controller 25 that outputs a drive signal 61 to turn ON the power switch Q1 in order to control an output voltage, Vout, across a load 24. A primary/secondary isolation circuit 30 provides an output voltage feedback signal that approximates the output voltage across load 24. An error voltage sense circuit 31 generates an error voltage from inputs that include a reference voltage, VREF, as well as the output voltage feedback signal from primary/secondary isolation circuit 30. This error voltage is used by the controller 25 for regulating the ON time of the power switch Q1.
Obtaining the output voltage feedback signal from the secondary side of the converter, as shown in FIG. 1, offers the potential of accurate regulation performance, but necessarily increases the complexity and cost of the control system. If a primary-side feedback system were used instead, the output voltage feedback signal would be obtained from the primary side of power transformer T1, reducing cost and complexity of the control system, but introducing difficulties with regulation accuracy.
In particular, a reflected voltage across the primary winding 20 is proportional to the output voltage across the load 24 minus a voltage drop produced by resistive and other losses in the secondary circuit, including losses across the rectifying diode D1. These losses will vary, depending upon the current drawn by the load and other factors. Hence, measuring the output voltage through the reflected flyback voltage is problematic, as parasitic losses act as a corrupting signal that cannot be removed by filtering. As such, prior art primary-side feedback systems, such as that disclosed in U.S. Pat. No. 5,438,499, which depends upon the reflected voltage, are challenged to provide good voltage regulation.
Accordingly, there is a need in the art for power converters having primary-only feedback that achieves regulation performance traditionally obtainable with secondary feedback, while preserving the intended simplicity and cost benefits of primary-only feedback.